The present invention relates to improving the efficiency of amplifiers and, in particular, a new and efficient front-end topology for radio frequency power amplifiers.
Battery power consumption (which translates to “talk time”) is a major consideration in wireless mobile terminals like cellular or cordless phones. Most of the power in such devices is consumed by the radio frequency (RF) power amplifier, or RF PA. The efficiency of an RF PA is defined as the RF output power divided by the DC power consumed from the battery. RF PAs are designed to provide a certain maximum RF power (Pmax) with required linearity. As a result, the efficiency of an RF PA is generally higher at Pmax. However, mobile devices typically spend most of the time transmitting at power levels which are much lower than Pmax. This happens, for example, when the mobile device is relatively close to a base station. Unfortunately, the efficiency of mobile devices under these conditions is typically very poor.
A variety of approaches to solving this issue have been proposed. According to one approach, the RF PA in the output power stage is dynamically re-biased to improve efficiency at lower power levels. However, such power stages are designed to handle high power levels and are therefore optimized for high power operation. Thus, when they are re-biased for low power operation, they tend to operate less efficiently than smaller, low power stages which are optimized for such power levels.
According to another approach in which power amplification is accomplished in multiple stages, a low power transmit path to a second antenna is provided which is driven by a pre-amplifier which is in series with or is part of the main RF PA. During low power operation, the main RF PA is turned off or quiesced, thus reducing power consumption and improving device efficiency. However, because this approach relies on amplification in the main transmit path, the low power transmit path remains on during higher power operation, representing an undesirable inefficiency. In addition, the pre-amplifier which drives the low power transmit path is not optimized for that purpose.
Thus, while existing approaches to improving RF PA efficiency at low power levels have made some inroads, there is still considerable room for improvement.